Báo cáo khoa học: Pulchellin, a highly toxic type 2 ribosome-inactivating protein from Abrus pulchellus Cloning, heterologous expression of A-chain and structural studies ppt

Pulchellin, a highly toxic type 2 ribosome-inactivating protein from Abrus pulchellus Cloning, heterologous expression of A-chain and structural studies Andre ´ L. C. Silva 1 , Leandro S. Goto 1 , Anemari R. Dinarte 2 , Daiane Hansen 3 , Renato A. Moreira 4 , Leila M. Beltramini 1 and Ana P. U. Arau ´ jo 1 1 Centro de Biotecnologia Molecular Estrutural, Instituto de Fı ´ sica de Sa˜o Carlos, Universidade de Sa˜o Paulo, Brazil 2 Fundac¸a˜o Hemocentro de Ribeira˜o Preto, Brazil 3 Universidade Federal de Sa˜o Paulo-EPM, Brazil 4 Universidade Federal do Ceara ´ , Brazil Ribosome-inactivating proteins (RIPs; EC 3.2.2.22) are RNA N-glycosidases that depurinate the major ribo- somal RNA (rRNA), thus damaging ribosomes and arresting protein synthesis [1]. RIPs are found predom- inantly in higher plants, but are also present in algae [2], fungi [3] and bacteria [4]. They vary greatly in their physical properties and cellular effects [5]. Based on the structural properties and their corresponding genes, RIPs have been classified as types 1, 2 and 3 [6]. Type 2 RIPs, like ricin and abrin, are highly toxic heterodimeric proteins that consist of a polypeptide with RIP activity (A-chain) linked to a galactose- binding lectin (B-chain) via a disulfide bond [7]. The A-chain is the catalytic subunit that exhibits rRNA N-glycosidase activity by removing a specific adenine residue from a conserved loop (ricin ⁄ sarcin loop) of the largest RNA in the ribosome [8]. This modification induces a conformational change that prevents binding Keywords abrin; lectin; ribosome-inactivating protein; RNA N-glycosidase Correspondence A. P. U. Arau ´ jo, Grupo de Biofı ´ sica Molecular e Espectroscopia, Instituto de Fı ´ sica de Sa˜o Carlos, Universidade de Sa˜o Paulo, Caixa Postal 369, CEP 13560-970, Sa˜o Carlos, SP, Brazil E-mail: anapaula@if.sc.usp.br (Received 15 October 2004, revised 6 December 2004, accepted 5 January 2005) doi:10.1111/j.1742-4658.2005.04545.x Pulchellin is a type 2 ribosome-inactivating protein isolated from seeds of the Abrus pulchellus tenuiflorus plant. This study aims to obtain active and homogeneous protein for structural and biological studies that will clarify the functional aspects of this toxin. The DNA fragment encoding pulchellin A-chain was cloned and inserted into pGEX-5X to express the recombinant pulchellin A-chain (rPAC) as a fusion protein in Escherichia coli. The deduced amino acid sequence analyses of the rPAC presented a high sequential identity (> 86%) with the A-chain of abrin-c. The ability of the rPAC to depurinate rRNA in yeast ribosome was also demonstrated in vitro. In order to validate the toxic activity we promoted the in vitro association of the rPAC with the recombinant pulchellin binding chain (rPBC). Both chains were incubated in the presence of a reduced ⁄ oxidized system, yielding an active heterodimer (rPAB). The rPAB showed an apparent molecular mass of  60 kDa, similar to the native pulchellin. The toxic activities of the rPAB and native pulchellin were compared by intra- peritoneal injection of different dilutions into mice. The rPAB was able to kill 50% of the tested mice with doses of 45 lgÆkg )1 . Our results indicated that the heterodimer showed toxic activity and a conformational pattern similar to pulchellin. In addition, rPAC produced in this heterologous sys- tem might be useful for the preparation of immunoconjugates with poten- tial as a therapeutic agent. Abbreviations CD, circular dichoism; GST, glutathione S-transferase; LD 50 , median lethal dose; PAC, pulchellin A-chain; RIP, ribosome-inactivating protein; rPAB, recombinant pulchellin heterodimer; rPAC, recombinant pulchellin A-chain; rPBC, recombinant pulchellin B-chain. FEBS Journal 272 (2005) 1201–1210 ª 2005 FEBS 1201 of elongation factor 2 (EF2) to the ribosome, resulting in cell death due to protein synthesis arrest [9]. The B-chain has lectin properties, preferentially binding to galactosyl-terminated glycoproteins on the surface of eukaryotic cells leading to endocytosis. It also facili- tates A-chain penetration of the lipid bilayer and entrance into the cytosol [10]. Despite toxic activity, one group of type 2 RIPs is much less toxic to cells and animals, but shares structural and enzymatic prop- erties with highly toxic RIPs. This group has been named nontoxic type 2 RIPs [11]. There has been considerable interest in RIPs due to their potential role in the development of therapeutic agents. Perhaps the most promising approach to apply- ing RIPs in therapy is the use of immunotoxins in which the toxic A-chain is linked to antibodies directed toward specific cells [12,13]. Several immunotoxins derived from RIPs have been made and assayed against specific target cells in vitro and in vivo [14,15]. In addition, RIPs also display antiviral [16], antibacte- rial [17] and antifungal [18] activities. The apparent defense role against pathogens also extends to insect pests [19,20]. Abrus pulchellus tenuiflorus (Leguminosae-Papiliono- ideae) seeds contain a highly toxic protein named pulchellin. Pulchellin is a type 2 RIP that exhibits specificity for galactose and galactose-containing struc- tures, agglutinates human and rabbit erythrocytes, and kills mice and the microcrustacean Artemia salina at very low concentrations [21]. Here we report the clo- ning of pulchellin A-chain (PAC), its cDNA character- ization, expression of recombinant toxic A-chain (rPAC) in Escherichia coli, and the in vitro association of the rPAC and recombinant pulchellin binding chain (rPBC) [22], which produces an active heterodimer. We also performed structural studies of the recombinant proteins using circular dichroism spectroscopy. The cloning process will enable the production of soluble and active homogeneous protein, which is desirable to the study of its use in immunotherapy. Comparison of the primary sequences of type 2 RIPs and their structural characterization will clarify small differences that significantly change the citotoxity of such proteins, making them more appropriate for therapeutic use. Results Isolation and cloning of the pulchellin A-chain gene fragment Clones of several RIP-2 toxins, such as ricin and abrin have been obtained in other laboratories and shown to belong to a multigene family. Also, as with other plant lectin genes, these genes contain no introns [30–32]. Thus, our initial cloning strategy was based on the assumption that a similar situation also occurs in pul- chellin from A. pulchellus based on its phylogenetic closeness to abrin. Using degenerated primers, it was possible to amplify the fragment corresponding to the A-chain (active) and part of the B-chain (binding). After PCR, the amplified product was  970 bp, as predicted based on taxonomic proximity exhibited between pul- chellin and abrin. The genomic sequence obtained was submitted to homology search using blast software, which gave a nucleotide identity of 84% to abrin-c A-chain precursor from Abrus precatorius. Based on the cloned sequence, specific primers were designed to obtain 5¢-end sequence information via 5¢ RACE. As expected, this amplification product revealed a band around 450 bp with a high identity to the preproabrin gene of A. precatorius. Taken together, the results of genomic cloning and 5¢ RACE, indicated a 34-amino acid N-terminal leader peptide, 251 residues corresponding to pulchellin A-chain and a small linker peptide (14 residues) join- ing the A- and B-chains. As it was found that the Glu-Asp-Arg-Pro-Ile N-terminal sequence of native pulchellin A-chain after the amino acid sequence is very similar to that reported for the N-terminus of the abrin-c A-chain, it was possible to define the first amino acid of the mature PAC. Comparing the amino acid sequences with that of the abrin-c A-chain precur- sor from A. precatorius , similarities of 88, 86 and 93%, were found, respectively, for each region. The presence of both leader and linker peptides, as other type 2 RIPs, is strong evidence that pulchellin is also synthes- ized as a single chain precursor. The N-terminal leader sequence directs the immature precursor to the endoplasmic reticulum [33] and the linker peptide has been reported as a signal leading the toxin to the vacuoles [34]. Both the N-terminal leader and linker peptide are post-translationally excised resulting in an active toxin comprising two mature subunits. The overall sequence homology of the pul- chellin linker peptide is high, differing in only one amino acid residue among 14 present on the abrin-c linker, possibly suggesting the same biological roles for the sequences. Expression, purification and characterization of the recombinant pulchellin A-chain From A. pulchellus genomic DNA, the fragment enco- ding the mature PAC was amplified by PCR using a Pulchellin A-chain: cloning and structural studies A. L. C. Silva et al. 1202 FEBS Journal 272 (2005) 1201–1210 ª 2005 FEBS new set of primers giving rise to a product of  850 bp. The deduced amino acid sequence of this gene fragment showed a high identity to abrin-c (86%), abrin-a (78%) and ricin (38%) A-chain sequences (Fig. 1). The PAC sequence encodes a mature protein with a predicted molecular mass of around 29 kDa and a theoretical isoelectric point of 5.5. Alignment of the deduced amino acid sequences shows that all residues involved in the active site as described for abrin-a, abrin-c and ricin are conserved in the sequence reported here. Recent analyses of the crystal structures of ricin, trichosanthin, pokeweed antiviral protein, momordin and abrin-a indicate that the overall architecture of the active site cleft remains constant in all these proteins [10,35]. In addition, the sequence of PAC presented only one cysteine residue that should be involved in the interchain disulfide bridge. The DNA fragment encoding PAC was inserted into a pGEX 5X-1 vector (Amersham-Pharmacia) to express the recombinant A-chain as a protein fusion with glutathione S-transferase (GST). Escherichia coli AD 202 harboring pGEX-rPAC was used to produce soluble recombinant fusion protein with the predicted molecular mass ( 60 kDa) (Fig. 2A). The fusion pro- tein was purified from the cell lysate by affinity chro- matography on a glutathione–Sepharose column. After elution, the fusion protein was submitted to Factor Xa cleavage for 16 h, at 12 °C. Free recombinant pul- chellin A-chain (rPAC) was purified in an additional chromatographic step in a Mono-Q ion-exchange column. The yield of the rPAC soluble protein was of  3mgÆL )1 of the Luria–Bertani media culture. The rPAC was homogeneous upon analysis on 15% SDS ⁄ PAGE, with an apparent molecular mass of 29 kDa (Fig. 2B). The rPAC was also submitted to immunodetection using polyclonal antibodies (anti- native pulchellin), which recognized the recombinant protein (Fig. 2C). RNA N-glycosidase activity of the rPAC An RNA depurination test was used to confirm the in vitro enzymatic activity of rPAC. Figure 3 shows an Fig. 1. Deduced amino acid sequence of recombinant pulchellin A-chain (rPAC) aligned to abrin-a, abrin-c and ricin (RTA) A-chains. Conserved amino acids are highlighted in gray. rPAC residues involved in the potential active site cleft, as predicted by homology to RTA, abrin-a and abrin-c A-chains, are bold and indicated by *. The cysteine residue (indicated by fl), also due to homology, should be involved in an interchain disulfide bond. A. L. C. Silva et al. Pulchellin A-chain: cloning and structural studies FEBS Journal 272 (2005) 1201–1210 ª 2005 FEBS 1203 ethidium bromide-stained electrophoresis gel of anil- ine-treated yeast ribosomal RNA incubated with dif- ferent amounts of rPAC and native pulchellin (as positive control). Aniline treatment of rRNA from yeast ribosomes incubated with RIP at 10, 5 and 1 ng released a fragment of  370 nucleotides. In contrast, incubation of ribosome with 0.1 ng did not result in depurination. The depurination assay performed in the absence of rPAC or native pulchellin also failed to generate the RNA fragment. Taken together, these results suggest that the rPAC possesses RNA N-glyco- sidase activity just like the native pulchellin. In vitro association of rPAC and rPBC In an attempt to check the toxic activity of the rPAC in vivo, a protocol was used to obtain a functional heterodimer (named rPAB). The in vitro association of the two pulchellin subunits (expressed separately) was achieved by using an oxidized ⁄ reduced system as des- cribed in Experimental procedures. rPBC, obtained after the refolding process [22], and rPAC were pooled and incubated in 50 mm Tris ⁄ HCl buffer 100 mm NaCl, pH 8.0. Formation of the active rPAB hetero- dimer could be detected after 2 h incubation (Fig. 4A). At 4 °C, a plateau of recombinant heterodimer forma- tion was reached  48 h after the onset of the experi- ment. The yield of the rPAB association process was 0.2 mg, corresponding to 20% of the total theoretically obtainable heterodimer. After association, the protein was loaded into a CentriPrep (30 000 cut-off, Milli- pore) and dialofiltrated against the incubation buffer to separate the heterodimer from free rPAC and rPBC. Figure 4(B) shows the purity of the rPAB heterodimer after dialofiltration, under reducing (lane 1) and non- reducing (lane 2) conditions in SDS ⁄ PAGE silver- stained. An apparent molecular mass of  59 kDa for ABC Fig. 2. (A) A-chain expression analysis in SDS ⁄ PAGE, 15%. Lane 1, molecular mass marker; lanes 2 and 3, total proteins from E. coli AD 202–pGEX-rPAC not induced and induced by 0.4 mM isopropyl thio-b-D-galactopyranoside, respectively; lane 4, soluble fraction from cellu- lar lysates after sonication; lane 5, insoluble fraction; lane 6, fusion protein (A-chain plus GST) eluted from affinity resin. (B) rPAC purification analysis in SDS ⁄ PAGE, 15%. Lane 1, molecular mass marker; lane 2, fusion protein (GST + PAC) after Factor Xa cleavage; lane 3, samples eluted from the major peak of the Mono-Q, corresponding to the rPAC; lane 4, fraction corresponding to GST. (C) Western blot analysis using rabbit polyclonal antibodies against native pulchellin. Lane 1, rPAC; lane 2, native pulchellin. Fig. 3. N-glycosidase activities of rPAC and native pulchellin. Yeast ribosomes (20 lg) were incubated with different amounts (10, 5, 1 and 0.1 ng) of rPAC and native pulchellin for 1 h at 25 °C. The rRNAs were extracted and treated with 1 M aniline-acetic for 4 min at 60 °C. Samples were analyzed by denaturing agarose–formamide gel electrophoresis and staining with ethidium bromide. Yeast ribo- somes samples treated with rPAC (lanes 1–4), native pulchellin (lanes 5–8) and without treatment (negative control) (lanes 9–10) are shown. The arrow indicates the position of the small RNA frag- ment released upon aniline treatment of rRNA. +, presence of anil- ine treatment; –, absence of aniline treatment. Pulchellin A-chain: cloning and structural studies A. L. C. Silva et al. 1204 FEBS Journal 272 (2005) 1201–1210 ª 2005 FEBS the heterodimer is expected because the molecular mas- ses of rPAC and rPBC, are  29.2 and 29.8 kDa [22], respectively. The native pulchellin has an apparent molecular mass of 60 kDa [21] due to the native glyco- sylation process [36]. Circular dichroism and biological activity of the rPAB heterodimer Circular dichroism (CD) measurements and biological tests were used to investigate the similarity between the native pulchellin and the rPAB heterodimer. Figure 5 shows the far-UV CD spectra of rPAC, rPBC, rPAB and native pulchellin. CD analyses for the rPAC sample showed a protein profile with predominance of a-helical elements [37]: two negative bands at 222 and 208 nm and a positive peak at 196 nm. The CD spec- trum shape of refolded rPBC showed one maximum band at 231 nm, two minima at 214 and 206 nm, and a negative to positive crossover at 199 nm. This spec- trum showed that the b-sheet was the predominant component present in rPBC secondary structure. When compared, both native pulchellin and rPAB hetero- dimer presented very similar CD spectra. The biological activity of the rPAB heterodimer in terms of lethal dose (LD 50 ) values is given in Fig. 6. After 48 h, the rPAB was able to kill 50% of mice tes- ted with a dose of 45 lgÆkg )1 , which was a little less toxic than the lethal dose found for native pulchellin AB Fig. 4. In vitro association of rPAC with rPBC. (A) rPAC was incubated with rPBC in the presence of a reduced ⁄ oxidized cysteine system at 4 °C for 48 h. The reaction prod- ucts were analyzed using 15% SDS ⁄ PAGE and were silver-stained. Lane M, protein marker; numbered lanes correspond to incubation times. rPAB heterodimer appears as an additional band of  60 kDa after 2 h incubation (lanes 2–48). (B) rPAB hetero- dimer after dialofiltration, under reducing (lane 1) and nonreducing (lane 2) conditions. Fig. 5. CD spectra of recombinant pulchellin A-chain (rPAC), recom- binant pulchellin B-chain (rPBC), recombinant pulchellin (rPAB) and native pulchellin. Spectra were obtained from each protein at a con- centration of 0.3 mgÆmL )1 in 20 mM Tris ⁄ HCl, pH 8.0. Measure- ments were performed using quartz cuvettes of 1 mm path length and recorded from 195 to 250 nm as the average of 16 scans at 25 °C. buffer 100 90 80 70 60 50 Death (%) 40 30 20 10 0 15 30 Dose (µ g.Kg –1 animal) 45 50 60 Pulchellin rPAB rPAC rPBC Fig. 6. Lethal activity determined by intraperitoneal injection in mice using different concentrations of recombinant pulchellin A-chain (rPAC), recombinant pulchellin B-chain (rPBC), recombinant pulchel- lin (rPAB) and native pulchellin (as positive control). The buffers of each protein were used as negative controls. Groups of six animals and different doses of each protein were prepared. Each group rep- resented a dose and the toxic effects were determined after 48 h. A. L. C. Silva et al. Pulchellin A-chain: cloning and structural studies FEBS Journal 272 (2005) 1201–1210 ª 2005 FEBS 1205 (30 lgÆkg )1 ). Sublethal doses also lead to animal death some days later until the end of experiments. Although this value is higher than found for other similar toxins [38], the toxic effects observed agree with those induced by type 2 RIPs. The structural and biological properties determined for the rPAB heterodimer showed that this protein presents similar behavior to that of the native pulchellin. Discussion Pulchellin, a type 2 RIP isolated from A. pulchellus seed, is a potent plant toxin, similar to abrin and ricin. Cloning of the coding gene from pulchellin A-chain will greatly facilitate the understanding of the protein structure and function, and lay a solid foundation for its application. This study reports the cloning and characterization of the A-chain gene that encodes the toxic chain of pulchellin. rPAC was expressed in a soluble form, preserving its structure and biological activity. Its DNA sequence has very high identity with (93.0%) and a similar size to (251 bp) abrin-c A-chain [39]. The molecular mass of rPAC (29 kDa) is consistent with that reported for native pulchellin A-chain [21]. rPAC was found to be highly homologous to other type 2 RIPs [30,40]. As shown in Fig. 2, rPAC shows a high sequence homo- logy to A-chains from abrins. In the four RIPs listed, there is one conserved cysteine residue close to the C-terminal of the A-chains, which allows formation of one interchain bond with another conserved cysteine residue in their respective B-chains. The active RNA N-glycosidase sites of abrin-a, abrin-c and ricin are composed of five invariant residues (Tyr74, Tyr113, Glu164, Arg167 and Trp198 in abrin-a and abrin-c, and Tyr115, Tyr158, Glu212, Arg215 and Trp246 in ricin) and another five conserved residues (Asn72, Arg124, Gln160, Glu195 and Asn196 in abrin-a and abrin-c and Asn78, Arg134, Gln172, Glu208 and Asn209 in ricin) [30,35]. The alignment of amino acid sequences shows that all residues involved in the active site cleft of abrin-a, abrin-c and ricin are totally con- served in the rPAC sequence. The N-glycosidade activity assays showed that rPAC was enzymatically active. RIP-mediated depurination of the large ribosomal subunit RNA results in a sus- ceptibility of the RNA sugar–phosphate backbone to hydrolysis at the depurination site, which leads to the release of a small fragment of 130–400 nucleotides from the 3¢-end of the rRNA [41,42]. This fragment is diagnostic of RIP-catalyzed depurination and is readily observed following agarose–formamide gel electro- phoresis [43]. rPAC (1 ng) was able to cleave the N-glycosidic bond of yeast rRNA, releasing an RNA fragment of  370 nucleotides after treatment with aniline, as did native pulchellin. Thus, this activity can be attributed to conserved residues that form the active site of RNA N-glycosidase in rPAC. Stirpe et al. [44] showed that a fragment of  400 nucleotides arises from removal of A3024 in yeast 26S rRNA when incu- bated with ricin. Using the intraperitoneal toxicity test to compare the potency and activity of rPAB heterodimer and native pulchellin, no significant differences between the recombinant heterodimer and native protein were found. Neither rPAC nor rPBC had a toxic effect on mice in the dosage range used. Thus, it is clear that in vivo poisoning occurs only if the whole heterodi- meric protein (rPAB) is administered. This activity was expected because the CD results show that rPAB has the same CD profile and consequently, has a secon- dary structure fold similar to the native pulchellin. Our results are in accordance those of with Eck et al. [45], who compared the toxic activities of single chains from plant mistletoe lectin (pML) with the recombinant mistletoe lectin heterodimer (rML), concluding that both lectin and rRNA N-glycosidase activities are pre- requisites for cytotoxic effect on target cells. In addition, our results also suggest that glycosyla- tion is not essential for heterodimer internalization because the rPAB heterodimer is derived from biosyn- thesis in E. coli (therefore it is not glycosylated) and showed toxicity similar to that of native pulchellin. In fact, the absence of glycolysation has advantages when using the A-chains in immunotoxins. For exam- ple, deglycosylated ricin A-chain (dgA) immunotoxins greatly reduced the levels of nonspecific uptake by the liver and concomitantly increased tumor-specific local- ization [46,47]. Regarding the therapeutic use of immunotoxins, an important consideration for immunoconjugate assembly is the nature of the linkage between anti- body and RIP [47]. A disulfide bridge is usually thought to be essential for maximal cytotoxicity. Most type 1 RIPs do not have any free cysteine resi- dues [48], which implies the need for modification of both antibody and RIP with chemical agents to pro- duce the disulfide bond. Fortunately, rPAC has one free cysteine located in the C-terminal region and can directly form a disulfide bond with an activated antibody thiol group via a disulfide-exchange reac- tion. Therefore, rPAC is easily produced in a heterologous system and it might be useful for the preparation of immunoconjugates with great poten- tial as a chemotherapeutic agent for the treatment of cancer [11,47,49] and AIDS [50,51]. Pulchellin A-chain: cloning and structural studies A. L. C. Silva et al. 1206 FEBS Journal 272 (2005) 1201–1210 ª 2005 FEBS Experimental procedures Materials E. coli DH5-a (Promega, Madison, WI, USA) was used for plasmid amplification and E. coli ad 202 strain (Novagen, Madison, WI, USA) was used to express the gene. pGEX 5X-1 expression vector was purchased from Amer- sham-Pharmacia Biotech (Piscataway, NJ, USA). Isopropyl thio-b-d-galactoside was purchased from Sigma (St. Louis, MO, USA). Oligonucleotide synthesis was produced by Gibco BRL (Rockville, MD, USA). Restriction endonuc- leases, and DNA ladders were obtained from Promega. Factor Xa protease was purchased from Biolabs (Beverly, MA, USA). All other chemicals used were analytical grade. Plant material and nucleic acid isolation from A. pulchellus A. pulchellus tenuiflorus subspecies were cultivated in our laboratories to produce the necessary tissues for nucleic acid extractions. Approximately 1.5 g of leaves were frozen and ground to powder in liquid nitrogen. Genomic DNA was further isolated using a plant genomic DNA isolation Floraclean kit (Qiagen, Valencia, CA, USA), following the manufacturer’s instructions. Total RNA was isolated from immature A. pulchellus seeds, previously frozen in liquid nitrogen, using the RNA- easy Plant Mini Kit (Qiagen). The total RNA was quanti- fied at 260 nm (Hitachi U-2000 spectrophotometer) and 2 lg was used to 5¢RACE. Genomic cloning Degenerate primers were designed based on the amino acid sequence conservations along the preproabrin gene (MED- LINE 91266957) and were used for genomic amplifications. Their design was based on the A. precatorius codon table, trying to minimize the degeneration at their 3¢ ending. A pair of degenerate primers (abrin 1: 5¢-ACTGAAGGTGCC ACTTCACAAAGCTAYAARCARTT-3¢; abrin 3: 5¢-GGT TAAACACTTCCCGTTGGACCTDATNGT-3¢) was cho- sen to represent the possible coding sequences of the con- served N-terminus of the pulchellin A- and B-chains. Thus, the expected amplified product could represent the major sequence encoding the A-chain and an additional fragment encoding part of the B-chain. The primers described above were used in a PCR con- taining the A. pulchellus genomic DNA as a template. The reaction mixture included: 100 pmol of each primer; 1.0 lg of A. pulchellus DNA template; 200 lm for each dNTP; 1· PCR buffer (Amersham-Pharmacia Biotech); 2.5 U Taq DNA polymerase (Amersham-Pharmacia Biotech) in a total volume of 50 lL. PCR was performed for: 1 cycle at 94 °C for 5 min; 30 cycles at 94 °C for 1 min, 45 °C for 1 min, and a primer extension for 1 min at 72 °C; and a final cycle at 72 ° C for 7 min. The products obtained by amplification were cloned in the pGEM-T easy vector (Promega), which was used to transform E. coli DH5-a competent cells. Sequencing The positive clones were sequenced in the ABI-Prism 377 (Perkin–Elmer) automatic sequencer following the manufac- turer’s instructions. The whole fragment was sequenced and submitted to a blast script data bank search [23]. RACE The 5¢ RACE was performed using Access RT-PCR Introductory System according to an adapted protocol previously described [24]. Terminal transferase (Life Technologies, Rockville, MD, USA) was used to add a homopolymer G-tail in the first strand for 5¢ RACE. Speci- fic primers were designed for this step based on DNA sequences obtained previously. Thus, the sequences of the primers used for 5¢ RACE were: 5¢-GGGCATCACGGA AGAAATAG-3¢ for a reverse transcription and 5¢-GC TCTAGAGCATTCGTCACATCGATACC-3¢ with 5¢-AA GGAATT(dC)14 for the following amplification. The ther- mal profile was 40 cycles of 96 °C for 1 min, 55 °C for 2 min, 72 °C for 3 min and a final extension for 10 min at 72 °C. The PCR products were analyzed on 1% agarose gels. Subsequently, the RACE reaction product was puri- fied and inserted into vector pGEM-T (Promega). One microliter of this mixture was used to transform E. coli EletroMax DH5a-E cells (Gibco-BRL) by electroporation. The positive clones were sequenced was already described. Pulchellin A-chain cloning and expression A new oligonucleotide set was then synthesized to amplify the pulchellin A-chain gene fragment from A. pulchellus (GenBank accession number AY781337) by PCR. The sequences of the synthetic oligonucleotides used for amplifi- cation were pulcA ⁄ BamHI (5¢-CG GGATCCAGGAGGAC CGGCCCATTGAATTTACTACTG-3¢, the BamHI restric- tion site included is underlined) and the reverse primer pulcA ⁄ NotI(5¢-ATAGTTTA GCGGCCGCTCAATTTGGC GGATTGCAGAC-3¢, NotI restriction site is underlined). The product obtained by amplification was inserted into pGEX 5X-1 (Amersham-Pharmacia Biotech). Briefly, amplification was carried out in a 50 lL reaction volume containing  625 ng genomic DNA, 100 pmol of each pri- mer, 0.2 mm dNTPs and 2 U of Deep Vent DNA poly- merase (Biolabs) in the PCR buffer recommended by the enzyme manufacturer. Cycling parameters were: 1 cycle at 96 °C for 5 min, 5 cycles (94 °C for 1 min, 57 °C for A. L. C. Silva et al. Pulchellin A-chain: cloning and structural studies FEBS Journal 272 (2005) 1201–1210 ª 2005 FEBS 1207 1.5 min and 72 °C for 1 min), 25 cycles (94 °C for 1 min, 60 °C for 1.5 min and 72 °C for 1 min) followed by 10 min at 72 °C to a final extension. Both amplified fragment and pGEX 5X-1 vector were digested with BamHI and NotI endonucleases and purified. Such digestion resulted in cohe- sive sticky ends able to directionally insert ligation, which was performed by a T4 DNA ligase reaction. E. coli DH5-a competent CaCl 2 cells were transformed with the recombinant plasmid (named pGEX-rPAC) by heat shock treatment [25]. The expression plasmid pGEX-rPAC was used to trans- form competent E. coli ad 202 strain. The transformed cells ad 202 pGEX-rPAC were grown at 37 °C in Luria–Bertani medium supplemented with kanamycin (50 lgÆmL )1 ) and cultured up to a cell density absorbance of A 600 ¼ 0.4–0.6. Once this density was reached, the expression of recombin- ant protein was induced with 0.4 mm isopropyl thio- b-d-galactopyranoside and carried out for 12 h at 20 °C. Before and after induction, cell aliquots were collected by centrifugation and analyzed by 15% SDS ⁄ PAGE [26]. The remaining cells were pelleted by centrifugation and resus- pended in 8 mL of 0.1 m pH 8.0 NaCl ⁄ P i buffer containing 1.0 mgÆmL )1 lysozyme. After 30 min incubation on ice, cells were disrupted by sonication and the lysate was clar- ified by centrifugation at 20 000 g. At this point, both pellet and supernatant were submitted to SDS ⁄ PAGE 15% to check the solubility of the recombinant pulchellin A-chain (named rPAC). The clear supernatant was used for the purification step. Purification of rPAC The supernatant obtained was applied to a 2 mL glutathi- one–Sepharose 4 Fast Flow (Amersham-Pharmacia) and the column was washed with 10 vol. of NaCl ⁄ P i . After this, 5 vol. of the elution buffer (50 mm de Tris ⁄ HCl, 10 mm of reduced glutathione, pH 8.0) were loaded and the recom- binant A-chain was collected. This recombinant protein was eluted, pooled and submitted to Fator Xa cleavage protocol followed by an additional chromatographic step in the Mono-Q HR 5 ⁄ 5 (1 mL). Western blot analysis rPAC was submitted at immuno- detection, after SDS ⁄ PAGE, onto nitrocellulose membranes (Protan, Keene, NH, USA), using a Bio-Rad electrotransfer cell, for 2 h at 110 V. Membranes were developed with a secondary antibody–alkaline phosphatase detection system (Promega), using rabbit polyclonal antibodies produced against native pulchellin. An antiserum titer of 1 : 5000 was used for all experiments. Assay of the N-glycosidase activity of rPAC The isolation of yeast (Pichia pastoris) ribosome was per- formed as previously described [27]. Yeast ribosomes (20 lg) were incubated at 25 °C for 1 h with different amounts of rPAC (0.1, 1, 5 and 10 ng) in buffer A (20 mm Tris ⁄ HCl pH 8.0, 100 mm NaCl) in a total volume of 20 lL. The reaction was stopped by the addition of 0.1% SDS. The rRNA was obtained by phenol–chloroform extraction and precipitated by the addition of 0.1 vol. 2 m NaOAc pH 6.0 and 2.5 vol. 100% ethanol. The reaction mixtures were frozen and the precipitated rRNA was pelleted by centrifugation at 13 000 g for 30 min at 4 °C. The pellets were washed once with 70% ethanol and dried for 20 min in a vacuum desiccator. rRNA (10 lg) was treated (for 4 min, at 60 °C) with 20 lLof1m aniline-acetic (pH 4.5) or 20 lLofH 2 O for nonaniline-treated controls. The reactions were stopped by the addition of 0.1 vol. of NH 4 OAc and 2.5 vol. of 100% ethanol and frozen before centrifugation for 1 h at 4 °C. The pellets were resuspended in 15 lL of 60% formamide ⁄ 0.1· TPE (3.6 mm Tris, 3 mm NaH 2 PO 4 , 0.2 mm EDTA) mix and run on a denaturing agarose–formamide gel electrophoresis. The RNA was visualized on a short-wave ultra-violet transilluminator. In vitro association of rPAC and rPBC The recombinant pulchellin heterodimer (named rPAB) was prepared by coupling isolated, rPAC and the recombinat pulchellin binding chain (rPBC). The rPBC was produced as described previously by Goto et al. [22]. For association of rPAC and rPBC, the two chains (0.5 mg of each chain) were incubated in 50 mm Tris ⁄ HCl buffer, 100 mm NaCl, pH 8.0 at 4 °C for 48 h. For the formation of interchain disulfide bridges, the reaction was incubated in the presence of a reduced ⁄ oxidized system (cysteine to cystine ratio 5 : 1). The association process was followed by 15% SDS ⁄ PAGE under nonreducing condi- tions. Silver staining was performed as described by Blum et al. [28]. Circular dichroism measurements CD spectra were recorded with a Jasco J-715 spectropola- rimeter over a wavelength range of 195–250 nm. Measure- ments were made in quartz cuvettes of 1 mm path length, recorded as an average of 32 scans. CD spectra were meas- ured in protein solutions of 0.3 mgÆmL )1 . CD spectra were obtained in millidegrees and converted to molar ellipticity. Secondary structure fractions were calculated from decon- volution of the CD spectra using the program selcon 3 [29] employing a database of 42 proteins. Biological activity in vivo of the rPAB The biological activity of the recombinant pulchellin was studied by measuring its toxic activity (in vivo). Toxic activ- ity was determined by intraperitoneal injection in mice using different doses (15, 30, 45, 50 and 60 l g Æ kg )1 of Pulchellin A-chain: cloning and structural studies A. L. C. Silva et al. 1208 FEBS Journal 272 (2005) 1201–1210 ª 2005 FEBS animal body mass) of recombinant pulchellin. Native pul- chellin, produced as described by Ramos et al. [21], rPAC and rPBC were used as controls. Groups of six animals and different doses of each protein were prepared. Each group represented a particular dose and each animal in the same group received the same dose of protein in proportion to their body mass. After injection of each dose, the toxic effects were determined after 48 h and acute LD 50 values were calculated. Acknowledgements We thank Dr Heloı ´ sa S. S. de Arau´ jo for N-terminal analysis, and Andressa P. A. Pinto for contributions to this study. 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Biochem Biophys Res Commun 17, 965–971. 51 Au TK, Collins RA, Lam TL, Ng TB, Fong WP & Wan DCC (2000) The plant ribosome inactivating pro- teins luffin and saporin are potent inhibitors of HIV-1 integrase. FEBS Lett 471, 169–172. Pulchellin A-chain: cloning and structural studies A. L. C. Silva et al. 1210 FEBS Journal 272 (2005) 1201–1210 ª 2005 FEBS . Pulchellin, a highly toxic type 2 ribosome-inactivating protein from Abrus pulchellus Cloning, heterologous expression of A- chain and structural studies Andre ´ L degeneration at their 3¢ ending. A pair of degenerate primers (abrin 1: 5¢-ACTGAAGGTGCC ACTTCACAAAGCTAYAARCARTT-3¢; abrin 3: 5¢-GGT TAAACACTTCCCGTTGGACCTDATNGT-3¢)